Although anal intercourse is a common mode of HIV-1 transmission, particularly for men who have sex with men, many fundamental and mechanistic questions remain. The long-term objective of this study is to understand the early events in HIV-1 rectal transmission and to gain new insights, which will potentially lead to new intervention strategies, including vaccines and topical microbicides optimized for rectal use. Studying HIV-1 vaginal transmission in the highly relevant SIV-rhesus macaque model has revealed important events critical for transmission, such as target cell availability, innate immune response, and inflammation at the portal of entry, all factors which play a key role in transmission. Because the anatomical/histological features and the physiological functions of vagina and cervix differ from the rectum, we hypothesize that the underlying mechanisms and timing of HIV-1 rectal transmission will significantly differ from those of vaginal-cervical transmission. Utilizing innovative approaches and an established conceptual framework from studies of vaginal transmission, we will test this hypothesis by investigating three specific aims: 1) determine the timing, location, spatial distribution and cell types of virus-infected cells at the portal of entry and in draining and distal lymphatic tissues; 2) determine relationships between virus and susceptible target cells (CD4+ T cells, macrophages, and dendritic cells) and between virus-infected cells and the mucosal innate immune response in the rectum shortly after intra-rectal SIV inoculation, thereby determining whether changes in target-cell availability play a critical role in local expansion and systemic spread of infection; and 3) determine the spatial relationships and ratio of virus-specific CD8+ T effector cells (E) and virus-infected-target cells (T) at the portal of entry and in other tissue compartments to extent of control of virus replication, using a novel procedure combining in-situ tetramers and in-situ hybridization The proposed research is significant as it is expected to reveal important mechanisms underlying rectal transmission and potentially provide guidance for designing anti-HIV-1 vaccines and topical microbicides.